Electron gun biasing system

ABSTRACT

An electron gun biasing system, particularly adapted for use in an electron microscope, in which a plurality of resistors are provided in series with the cathode of the electron gun. In parallel with each of the resistors is a photoconductor, and means are provided for selectively illuminating the various photoconductors so as to short out various resistors, and thus vary the bias. Typically, the resistors and photoconductors may be located inside a shielded box, and a plurality of light pipes may be directed through openings in the shielded box, so that the ends of the light pipes will be adjacent the photoconductors. A plurality of lamps may then be disposed at the ends of the light pipes outside the box, the lamps being selectively illuminated by a simple lamp illumination circuit.

Unite Dao et al.

States [54] ELECTRON GUN BIASING SYSTEM [72] Inventors: James Dao, Alameda; Nelson C. Yew, Los

[21] App]. No.: 79,419

[451 July 11,1972

3,474,245 10/1969 Kimura et al. ..250/49.5

Primary Examiner-William F. Lindquist Attorney-Townsend and Townsend ABSTRACT An electron gun biasing system, particularly adapted for use in an electron microscope, in which a plurality of resistors are provided in series with the cathode of the electron gun. ln

US. Cl. A, l K, parallel each of the resisofs is a photoconductor and 315/156, 323/21 means are provided for selectively illuminating the various [51] Int. Cl. ..H01 37/26, GOln 23/00 photoconductors so as to short out various resistors, and thus [58] Fleld of Search ..250/49.5 R, 49.5 A, 21 l K; vary the bias Typically. the resistors and photoconducmrs 315/]0 5 323/21 may be located inside a shielded box, and a plurality of light pipes may be directed through openings in the shielded box, so [56] Reerences cited that the ends of the light pipes will be adjacent the photocon- I UNITED STATES PATENTS ductors A plurality of lamps may then be disposed at the ends of the light pipes outside the box, the lamps being selectively lgggffgg i g Jr illuminated by a simple lamp illumination circuit. 3, arrison 3,449,619 6/l 969 Stalp ..315/l0 16 Claims, 1 Drawing Figure 24 F1 W XI X/ '2 24 A 9 2 KW l l l5 l3 AM HIGH ILLL J MINZTION VOLTAGE QZQEES" CIRCUIT SUPPLY '8 l PAT ENTEDJUL 1 1 I972 SUPPLY I5 13 FILAMENT HIGH VOLTAGE SUPPLY LAMP ILLUMINATION CIRCUIT ELECTRON GUN BIASING SYSTEM This invention relates to an electron gun biasing system, and more particularly, to an electron gun biasing system particularly adapted for use in an electron microscope.

Typically, an electron gun comprises a cathode and a grid, the grid being disposed in front of the cathode. In order to control or limit the beam current, a voltage difference or bias is applied between the cathode and grid, the grid being more negative than the cathode. in numerous applications, such as in an electron microscope, it is often desired to vary the beam current by varying the potential difference or bias between the cathode and the grid. Typically, this is accomplished by placing a plurality of resistors in series with the cathode, and providing a switch for selectively shorting out certain of the resistors, and thus removing them from the circuit. By shorting out the resistors, the voltage difference or bias between the cathode and grid will be reduced, and the beam current will thus be increased.

The foregoing electron gun biasing system suffers from many drawbacks and disadvantages. In particular, the electron gun is often maintained at a high voltage, thus requiring that the biasing resistors and switch be well isolated and insulated. Accordingly, the biasing resistors and switch are generally located in a shielded box, the switch being operated by an insulating rod extending from the box. This restricts the location of the bias control, as complex and expensive mechanical linkages are required if it is desired to control the bias from a location remote from the shielded box containing the resistors and switch.

According to the present invention, a plurality of resistors are provided in series with the cathode of the electron gun. In parallel with each of the resistors is a photoconductor, and means are provided for selectively illuminating the various photoconductors so as to short out various resistors, and thus vary the bias. Typically, the resistors and photoconductors may be located inside a shielded box, and a plurality of light pipes may be directed through openings in the shielded box, so that the ends of the light pipes will be adjacent the photoconductors. A plurality of lamps may then be disposed at the ends of the light pipes outside the box, the lamps being selectively illuminated by a simple lamp illumination circuit.

The foregoing electron gun biasing system is advantageous in that the lamp illuminating circuit may readily be located remote from the shielded box, so that the bias control may be conveniently located. Furthermore, the electron gun biasing system according to the present invention is more simple and inexpensive than prior art systems, particularly where remote location of the bias control is desired.

Accordingly, it is an object of the present invention to provide an electron gun biasing system in which the bias control may conveniently be located remote from the bias resistors.

Another object of the present invention is to provide an electron gun biasing system which is more simple and inexpensive than prior art electron gun biasing systems.

Yet another object of the present invention is to provide an electron gun biasing system particularly adapted for use in an electron microscope.

Still another object of the present invention is to provide an electron gun biasing system in which a photoconductor is in parallel with each of the resistors and means are provided to selectively illuminate the various photoconductors, so as to selectively short out certain of the resistors.

These and other objects, features and advantages of the present invention will be more readily apparent from the following detailed description of the present invention, with reference to the accompanying drawing.

The accompanying drawing is a diagrammatic representation, partially in schematic form, of a scanning electron microscope incorporating the electron gun biasing system according to the present invention.

Referring to the drawing, there is shown an electron-optical column A of a scanning electron microscope. Disposed at one end of the electron-optical column A, there is an electron source, shown generally at 10, which produces a beam of electrons 11 directed substantially along the axis of the electronoptical column. A plurality of magnetic coils 12 are disposed successively along the path of the beam of electrons 11, and function to focus and deflect the beam of electrons 11 upon a specimen 14, supported by a specimen stage 13 at the other end of the electron-optical column. An electron collector 15 is disposed adjacent specimen 14, and functions to collect the electrons reflected from, or emitted by, specimen 14 so as to produce a conventional scanning electron microscope display, with the assistance of suitable electronic circuitry (not shown).

The foregoing scanning electron microscope structure is old in the art, and is described herein for illustrative purposes only, it being understood that the electron gun biasing system according to the present invention may be employed in conjunction with other electron gun applications.

Electron source 10 comprises a cathode l6 and a grid 17 disposed in front of cathode l6. Cathode 16 is electrically connected to a filament supply 18, which functions to energize cathode 16 so as to cause cathode 16 to emit the beam of electrons 11.

Typically, specimen 14 is maintained at ground potential by the grounding of specimen stage 13, as indicated in the drawing. Thus, in order to provide an appropriate accelerating voltage differential between electron source 10 and specimen l4, electron source 10 is connected to a high voltage power supply 19, which produces a suitable negative voltage.

In particular, grid 17 is directly connected to the output of high voltage power supply 19. in order to control the beam current, cathode 16 is maintained at a slightly less negative voltage by an electron gun biasing system. A voltage differential will thus exist between cathode 16 and grid 17, grid 17 being more negative than cathode l6, tending to reduce or limit the current of beam 1 1.

According to the present invention, cathode 16 is connected in series with a plurality of resistors 20 to high voltage supply 19. In parallel with each of the resistors 20 is a photoconductor 21. Photoconductors 21 possess the property that their resistance will be dependent upon the amount of light incident thereon. in particular, photoconductors 21 will possess a high resistance in the dark, and a low resistance when light is incident thereon. Accordingly, when light is incident upon certain of the photoconductors 21, the resistors 20 in parallel therewith will be effectively shorted out, and thus removed from the circuit. Thus, by selectively illuminating photoconductors 21, it is possible to vary the resistance in series with cathode 16, and thus vary the bias of electron source 10.

In accordance with the present invention, resistors 20 and photoconductors 21 may be enclosed in an equi-potential shielded box 22, which functions to electrically isolate the high voltage circuitry, and thus protect operators of the scanning electron microscope. Shielded box 22 includes a plurality of apertures, an insulating light pipe 23 extending through each of the 50 apertures the ends thereof are in proximity with each of the photoconductors 21, respectively. Insulating light pipes 23 may, for example, comprise optical fibers, or, may simply comprise a transparent plastic rod such as that available under the trade name Lucite.

At the other end of each of the insulating light pipes 23, there is disposed a lamp 24. Each of the lamps 24 function to illuminate respective photoconductors 21 through light pipe 23, when energized. Thus, energization or de-energization of lamps 24 will vary the bias of electron source 10.

Lamps 24 are electrically connected to a lamp illumination circuit 25. Lamp illumination circuit 25 functions to selectively illuminate lamps 24 so as to obtain the desired bias of electron source 10. in particular, lamp illumination circuit 25 may typically comprise a source of electricity and a multi-position switch adapted to illuminate the various lamps 24, depending upon the position thereof.

In operation, the operator of the scanning electron microscope may obtain the desired biasing of the electron source by causing lamp illumination circuit 25 to energize certain of the lamps 24. In particular, those lamps 24 which are illuminated will cause the photoconductors 21 associated therewith to assume a low resistance state, in which the resistors in parallel therewith are efiectively shorted out. This, in turn, will decrease the resistance in series with cathode 16, thus causing a smaller voltage difference between cathode 16 and grid 17. This reduction of voltage bias would result in an increase in beam 11 current. Thus, as more of the lamps 24 are illuminated, the beam current of the electron beam emitted by electron source 10 will increase. Conversely, the de-energization of lamps 24 will result in a decrease in beam 11 current.

The electron gun biasing system according to the present invention is advantageous in that the high voltage circuitry may be substantially enclosed in a shielded box, thus minimizing the danger of electrical shock. Furthermore, lamp illumination circuit may readily be located remote from shielded box 22, the only necessary interconnections therebetween comprising low voltage wiring for the illumination of lamps 24. Thus, the controls operative to adjust the electron gun biasing, and thus the beam current, may readily be located at any desired position, regardless of the location of shielded box 22. Furthermore, since the high voltage switching of resistances is accomplished electronically, rather than mechanically, the reliability and life of the apparatus is substantially improved while the cost and complexity is substantially reduced.

While a particular embodiment of the present invention has been shown and described, it is to be understood that adaptations and modifications may be made without departing from the true spirit and scope of the present invention, as set forth in the claims.

What is claimed is:

1. In an electron gun having a cathode, a grid disposed adjacent said cathode, and a source of high voltage D.C., said grid being connected to said source of high voltage, the improvement comprising: a plurality of resistors connected end to end to form a series circuit solely of resistors, one end of said series resistors being connected to said cathode and the other end of said series resistors being connected to said source of high voltage, a plurality of photoconductors, each of said photoconductors being connected in parallel with one of said resistors, and means for selectively illuminating said photoconductors on demand.

2. Apparatus according to claim 1 further comprising a shielded box, said resistors and said photoconductors being disposed within said shielded box.

3. Apparatus according to claim 2 wherein said means for selectively illuminating said photoconductors comprises a plurality of lamps, means for selectively illuminating said lamps on demand, and means for optically coupling each of said lamps with one of said photoconductors.

4. Apparatus according to claim 3 wherein said lamps are disposed outside of said shielded box, said shielded box having a plurality of apertures therein and said optical coupling means comprising a plurality of light pipes, said light pipes passing through said apertures in said shielded box, one end of each of said light pipes being disposed adjacent one of said photoconductors and the other end of each of said light pipes being disposed adjacent one of said lamps.

5. An electron gun D.C. biasing system comprising a cathode, a grid disposed adjacent said cathode, a source of high voltage D.C., said grid being connected to said source of high voltage, a plurality of resistors connected end to end to form a series circuit solely of resistors, one end of said series resistors being connected to said cathode and the other end of said series resistors being connected to said source of high voltage, a plurality of photoconductors, each of said photoconductors being connected in parallel with one of said resistors, and means for selectively illuminating said photoconductors on demand.

6. Apparatus according to claim 5 further comprising a shielded box, said resistors and said photoconductors being disposed within said shielded box.

7. Apparatus according to claim 6 wherein said means for selectively illuminating said photoconductors comprises a plu rality of lamps, means for selectively illuminating said lamps on demand, and means for optically coupling each of said lamps with one of said photoconductors.

8. Apparatus according to claim 7 wherein said lamps are disposed outside of said shielded box, said shielded box having a plurality of apertures therein and said optical coupling means comprising a plurality of light pipes, said light pipes passing through said apertures in said shielded box, one end of each of said light pipes being disposed adjacent one of said photoconductors and the other end of each of said light pipes being disposed adjacent one of said lamps.

9. In an electron microscope having a housing containing a specimen, a cathode disposed within said housing, a grid disposed adjacent said cathode, a source of high voltage D.C., said grid being connected to said source of high voltage, means for focusing the electrons emitted by said cathode on said specimen and means for evacuating the interior of said housing, the improvement comprising: a plurality of resistors connected end to end to form a series circuit solely of re sistors, one end of said series resistors being connected to said cathode and the other end of said series resistors being connected to said source of high voltage, a plurality of photoconductors, each of said photoconductors being connected in parallel with one of said resistors, and means for selectively illuminating said photoconductors on demand.

10. Apparatus according to claim 9 further comprising a shielded box, said resistors and said photoconductors being disposed within said shielded box.

1 1. Apparatus according to claim 10 wherein said means for selectively illuminating said photoconductors comprises a plurality of lamps, means for selectively illuminating said lamps on demand, and means for optically coupling each of said lamps with one of said photoconductors.

12. Apparatus according to claim 11 wherein said lamps are disposed outside of said shielded box, said shielded box having a plurality of apertures therein and said optical coupling means comprising a plurality of light pipes, said light pipes passing through said apertures in said shielded box, one end of each of said light pipes being disposed adjacent one of said photoconductors and the other of each of said light pipes being disposed adjacent one of said lamps.

13. In a scanning electron microscope having a housing containing a specimen, a cathode disposed within said housing, a grid disposed adjacent said cathode, a source of high voltage D.C., said grid being connected to said source of high voltage, means for focusing the electrons emitted by said cathode on said specimen, means for scanning the electrons emitted by said cathode on said specimen, and means for evacuating the interior of said housing, the improvement comprising: a plurality of resistors connected end to end to form a series circuit solely of resistors, one end of said series resistors being connected to said cathode and the other end of said series resistors being connected to said source of high voltage, a plurality of photoconductors, each of said photoconductors being connected in parallel with one of said resistors, and means for selectively illuminating said photoconductors on demand.

14. Apparatus according to claim 13 further comprising a shielded box, said resistors and said photoconductors being disposed within said shielded box.

15. Apparatus according to claim 14 wherein said means for selectively illuminating said photoconductors comprises a plurality of lamps, means for selectively illuminating said lamps on demand, and means for optically coupling each of said lamps with one of said photoconductors.

16. Apparatus according to claim 15 wherein said lamps are disposed outside of said shielded box, said shielded box having a plurality of apertures therein and said optical coupling means comprising a plurality of light pipes, said light pipes passing through said apertures in said shielded box, one end of each of said light pipes being disposed adjacent one of said photoconductors and the other end of each of said light pipes being disposed adjacent one of said lamps. 

1. In an electron gun having a cathode, a grid disposed adjacent said cathode, and a source of high voltage D.C., said grid being connected to said source of high voltage, the improvement comprising: a plurality of resistors connected end to end to form a sEries circuit solely of resistors, one end of said series resistors being connected to said cathode and the other end of said series resistors being connected to said source of high voltage, a plurality of photoconductors, each of said photoconductors being connected in parallel with one of said resistors, and means for selectively illuminating said photoconductors on demand.
 2. Apparatus according to claim 1 further comprising a shielded box, said resistors and said photoconductors being disposed within said shielded box.
 3. Apparatus according to claim 2 wherein said means for selectively illuminating said photoconductors comprises a plurality of lamps, means for selectively illuminating said lamps on demand, and means for optically coupling each of said lamps with one of said photoconductors.
 4. Apparatus according to claim 3 wherein said lamps are disposed outside of said shielded box, said shielded box having a plurality of apertures therein and said optical coupling means comprising a plurality of light pipes, said light pipes passing through said apertures in said shielded box, one end of each of said light pipes being disposed adjacent one of said photoconductors and the other end of each of said light pipes being disposed adjacent one of said lamps.
 5. An electron gun D.C. biasing system comprising a cathode, a grid disposed adjacent said cathode, a source of high voltage D.C., said grid being connected to said source of high voltage, a plurality of resistors connected end to end to form a series circuit solely of resistors, one end of said series resistors being connected to said cathode and the other end of said series resistors being connected to said source of high voltage, a plurality of photoconductors, each of said photoconductors being connected in parallel with one of said resistors, and means for selectively illuminating said photoconductors on demand.
 6. Apparatus according to claim 5 further comprising a shielded box, said resistors and said photoconductors being disposed within said shielded box.
 7. Apparatus according to claim 6 wherein said means for selectively illuminating said photoconductors comprises a plurality of lamps, means for selectively illuminating said lamps on demand, and means for optically coupling each of said lamps with one of said photoconductors.
 8. Apparatus according to claim 7 wherein said lamps are disposed outside of said shielded box, said shielded box having a plurality of apertures therein and said optical coupling means comprising a plurality of light pipes, said light pipes passing through said apertures in said shielded box, one end of each of said light pipes being disposed adjacent one of said photoconductors and the other end of each of said light pipes being disposed adjacent one of said lamps.
 9. In an electron microscope having a housing containing a specimen, a cathode disposed within said housing, a grid disposed adjacent said cathode, a source of high voltage D.C., said grid being connected to said source of high voltage, means for focusing the electrons emitted by said cathode on said specimen and means for evacuating the interior of said housing, the improvement comprising: a plurality of resistors connected end to end to form a series circuit solely of resistors, one end of said series resistors being connected to said cathode and the other end of said series resistors being connected to said source of high voltage, a plurality of photoconductors, each of said photoconductors being connected in parallel with one of said resistors, and means for selectively illuminating said photoconductors on demand.
 10. Apparatus according to claim 9 further comprising a shielded box, said resistors and said photoconductors being disposed within said shielded box.
 11. Apparatus according to claim 10 wherein said means for selectively illuminating said photoconductors comprises a plurality of lamps, means for selectively illuminating said lamps on demand, and means for opticaLly coupling each of said lamps with one of said photoconductors.
 12. Apparatus according to claim 11 wherein said lamps are disposed outside of said shielded box, said shielded box having a plurality of apertures therein and said optical coupling means comprising a plurality of light pipes, said light pipes passing through said apertures in said shielded box, one end of each of said light pipes being disposed adjacent one of said photoconductors and the other of each of said light pipes being disposed adjacent one of said lamps.
 13. In a scanning electron microscope having a housing containing a specimen, a cathode disposed within said housing, a grid disposed adjacent said cathode, a source of high voltage D.C., said grid being connected to said source of high voltage, means for focusing the electrons emitted by said cathode on said specimen, means for scanning the electrons emitted by said cathode on said specimen, and means for evacuating the interior of said housing, the improvement comprising: a plurality of resistors connected end to end to form a series circuit solely of resistors, one end of said series resistors being connected to said cathode and the other end of said series resistors being connected to said source of high voltage, a plurality of photoconductors, each of said photoconductors being connected in parallel with one of said resistors, and means for selectively illuminating said photoconductors on demand.
 14. Apparatus according to claim 13 further comprising a shielded box, said resistors and said photoconductors being disposed within said shielded box.
 15. Apparatus according to claim 14 wherein said means for selectively illuminating said photoconductors comprises a plurality of lamps, means for selectively illuminating said lamps on demand, and means for optically coupling each of said lamps with one of said photoconductors.
 16. Apparatus according to claim 15 wherein said lamps are disposed outside of said shielded box, said shielded box having a plurality of apertures therein and said optical coupling means comprising a plurality of light pipes, said light pipes passing through said apertures in said shielded box, one end of each of said light pipes being disposed adjacent one of said photoconductors and the other end of each of said light pipes being disposed adjacent one of said lamps. 